Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, for example, light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, that is, front-end manufacturing, and back-end manufacturing. Front-end manufacturing involves the formation of a plurality of semiconductor die on the surface of a semiconductor wafer. Each semiconductor die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing, especially wafer-level or panel level packaging, typically involves providing an environmentally robust encapsulation or protection of the device, formation of broader pitch interconnect structures, testing and singulation of individual semiconductor die from the finished wafer or panel. The term “semiconductor die” as used herein refers to both the singular and plural form of the words, and accordingly can refer to both a single semiconductor device and multiple semiconductor devices.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller semiconductor die size can be achieved by improvements in the front-end process resulting in semiconductor die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
Back-end processing can often include use of one or more insulating or polymer layers, such as PBO. PBO is a polymer used in the electronics packaging industry as an inter-level dielectric in packaging applications such as wafer level chip scale packaging (WLCSP) applications. PBO, like other insulating and polymer layers, can be photosensitive or non-photosensitive.
Insulating and polymer layers that are photosensitive can be patterned using photolithography. Photolithography involves the deposition of light sensitive material, e.g., a layer of photosensitive PBO. A pattern is typically transferred from a form of photomask to the photosensitive material using light. In an embodiment, the portion of the photosensitive material subjected to light is removed using a developer chemistry, exposing portions of the underlying layer. In another embodiment, the portion of the photosensitive material not subjected to light is removed using a developer chemistry, exposing portions of the underlying layer. The portions of the photosensitive film remaining can become a permanent part of the device structure.
Insulating and polymer layers that are not photosensitive can be patterned using photolithography and subtractive etching. Photolithography in this case involves the deposition of light sensitive material, e.g., photoresist, over the layer to be patterned, e.g., a layer of PBO. A pattern is typically transferred from a form of photomask to the photoresist using light. In one embodiment, the portion of the photoresist pattern subjected to light is removed using a solvent, exposing portions of the underlying layer to be patterned. In another embodiment, the portion of the photoresist pattern not subjected to light is removed using a solvent, exposing portions of the underlying layer to be patterned. The remaining photoresist serves as a mask protecting portions of the underlying layer. The exposed portions of the underlying layer are then removed by a subtractive etch process, typically wet etching, plasma etching, or laser ablation. The process used for the subtractive etch must have good selectivity to the photoresist layer, i.e., it must etch the underlying PBO or polymer layer while leaving the photoresist mask intact. Following the subtractive etch, the remainder of the photoresist is removed, leaving behind a patterned layer which becomes a permanent part of the device structure.
After photo processing the insulating, polymer, or PBO layer (i.e. by coating, exposing, and developing the photosensitive PBO or by covering the non-photosensitive PBO with a photoresist and performing subtractive etching) the polymer is cured at high temperatures to optimize the final film properties, reliability, and performance of the device.
As practiced in the prior art, and as per vendor recommendations, PBO or polymer curing is generally performed in a box oven or in a vertical furnace in a controlled nitrogen (N2) environment that requires slowly increasing a temperature of the box oven or vertical furnace for the curing of the PBO or polymer. Two types of PBOs, polymers, or insulating layers are commonly available in the market today: (1) a high cure temperature version, here referred to as standard PBO, standard polymer, or standard insulating layer; and a low cure temperature version, referred to as low temperature PBO, low temperature polymer, or low temperature insulating layer. FIG. 1A illustrates a typical temperature profile 2 for curing standard PBO in a box oven or vertical furnace as known in the art. A first or ramp-up portion 4 of temperature profile 2 is the period in which temperature is increased from room temperature (about 20-25° C.) to a maximum curing temperature. During ramp-up portion 4, temperature is slowly increased at a rate of approximately 2.1° C. per minute. A top or peak portion 6 of temperature profile 2, in FIG. 1A shows a desirable curing temperature of about 340° C. is achieved and maintained for a period of approximately one hour or 60 minutes. Typical peak temperatures for curing standard PBO in box ovens range from approximately 320° C. to 340° C. A final or ramp-down portion 8 of temperature profile 2 shows that the temperature is slowly decreased at a rate of approximately 3.2° C. per minute until the PBO layer and box oven or vertical furnace has cooled from the curing temperature to room temperature.
FIG. 1B illustrates a typical temperature profile 10 for curing low temperature PBO in a box oven or vertical furnace as known in the art. A first or ramp-up portion 12 of temperature profile 10 is the period in which temperature is increased rapidly from room temperature (about 20-25° C.) to 100° C. The temperature is held at approximately 100° C. for a period of approximately 30 minutes, as indicated by second or constant portion 13 of temperature profile 10. Another ramp-up or third portion 14 of temperature profile 10 is the period in which temperature is increased from approximately 100° C. to a maximum curing temperature as indicated by top or peak portion 15. During ramp-up portion 14, temperature is slowly increased at a rate of approximately 1.67° C. per minute. Top or peak portion 15 of temperature profile 10, in FIG. 1B shows a desirable curing temperature of about 200° C. is achieved and maintained for a period of approximately one hour or 60 minutes. Typical peak temperatures for curing low cure PBO in box ovens range from 175° C. to 200° C. A final or ramp-down portion 16 of temperature profile 10 shows that the temperature is slowly decreased at a rate of approximately 2.2° C. per minute until the PBO layer and box oven or vertical furnace has cooled from the curing temperature to room temperature.
By slowing a rate at which temperature increases, particularly, for example, during ramp-up portion 4 of temperature profile 2 or during ramp up portion 14 of temperature profile 10, a contour or slope of the vias formed within the PBO layer is maintained and does not undesirably deform during heating or curing. As shown in FIGS. 1A and 1B, an entire cure cycle as practiced in the prior art generally requires multiple hours to complete, 4 to 5 hours being typical. See, e.g., HD8820 Process Guide, published by HD Microsystems (August 2005) relating to standard PBO treatment, and HD8930 Process Guide, published by HD Microsystems (May 2009) relating to low temperature PBO treatment. Curing of PBO using the conventional techniques described above with respect to FIGS. 1A and 1B is performed by major Outsourced Semiconductor Assembly and Test (OSAT) providers and WLCSP providers in the fabrication of WLCSPs such as, for example, Amkor, Advanced Semiconductor Engineering (ASE), Taiwan Semiconductor Manufacturing Company (TSMC), Siliconware Precision Industries Co. Ltd. (SPIL), and Stats-Chippac.